The herbicidal activity of aryloxyphenoxy propionic acids and derivatives thereof is well known in the art. Furthermore, optical isomers are often known to exhibit enhanced herbicidal activity over the corresponding racemates. For example, U.S. Pat. No. 4,531,969 has disclosed that the R-enantiomers of certain 2-(4-aryloxyphenoxy)propionic acids and certain derivatives thereof are distinguished by a considerably enhanced herbicidal action compared to the racemic modifications. Since reduced quantities of herbicide are required to achieve comparable levels of control, the application of mixtures enriched in the more efficacious R-enantiomer offers both economical and environmental advantages.
To exploit the agrinomic benefits of these advantages, it is necessary to efficiently resolve the racemic mixtures of the herbicides that are normally produced industrially and to accurately determine the ratio of the R- and S-enantiomers in the resulting composition.
Various methods for obtaining high concentrations of individual enantiomers are known. The most common method of resolution of a racemic modification involves its conversion by an optically active reagent into a mixture of diastereomers which can then be separated on the basis of their different physical properties. Diastereomers are generally separated by fractional crystallization, though occasionally by fractional distillation or chromatography. Once the diastereomers have been separated, they can be reconverted to the individual enantiomers and the optically active resolving agent can be recovered.
Unfortunately, the differences in physical properties of the diastereomers are rarely if ever great enough to effect a total separation with one crystallization. Usually repeated crystallizations must be used and the process is long and tedious. The same situation generally applies to differential boiling points and differential absorption with respect to distillation and chromatographic separations.